The Talmud in the beginning of the sixth chapter of Masechet Berachot seeks to find the source that blessings are required for food. At one point in this adventure, the Talmud concludes that there is one available exposition to teach us the requirement of blessings, but if that is the case then we would only know the requirement of an after-blessing but not a before-blessing. The Talmud then explains that we would be able to derive the requirement for a before-blessing using a kal vachomer from the requirement for an after-blessing:
אי נמי יליף גזרה שוה אשכחן לאחריו לפניו מנין הא לא קשיא דאתיא בקל וחומר כשהוא שבע מברך כשהוא רעב לא כל שכן
And even if he does avail himself of a gezerah shawah, while we are satisfied that a blessing is required after it, whence do we learn that it is required [before partaking]? — This is no difficulty. We derive it by argument a fortiori: If he says a blessing when he is full, how much more so ought he to do so when he is hungry?
This seems to be assuming that it is more likely for there to be a requirement for a before-blessing than for there to be a requirement for an after-blessing. Thus if there is a requirement for an after-blessing (based on the one available exposition) then there would certainly be a requirement for a before-blessing.
But if that is the case, then perhaps there is only a requirement for a before-blessing and not an after-blessing. How can we assign the one available exposition to be a source for an after-blessing? That exposition should be applied to require a before-blessing, since if we have a choice between which should be required it would be a before-blessing. Once that is true there should be no source left to tell us that an after-blessing is required (except by bread which has an explicit verse)!